This document shows you how to perform tests to determine the
best match impedance setting for an analog Foreign
Exchange Office (FXO), Foreign Exchange Station (FXS), or Direct Inward Dialing
(DID) voice port. The voice port connects to a voice switch such as a private
branch exchange (PBX), a telephone company (telco), or central office (CO).
With the judicious choice of the impedance setting for a voice port, you can
improve echo cancellation (ECAN) performance. You can also mitigate any audible
voice quality problems on the trunk.

This document assumes that the reader already has an operational voice
router configuration and that both inbound and outbound call scenarios function
as expected.� This document builds on the configuration of an analog voice
router that already works. The procedure in this document tunes the analog
voice ports for optimal impedance matching to the telco lines.

Cisco IOS® Software Release 12.3(11)T and later support the testing
features that this document discusses. The document discusses two different,
but related, testing features. Therefore, the document mentions specific Cisco
IOS Software releases only as necessary.

Where the document names specific hardware parts, the applicable
software versions are those which support the named hardware. Refer to these
documents for hardware and software compatibility matrices for analog FXO, FXS,
and DID voice products:

The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.

Assume the VoIP network topology that appears in this section for the
purpose of this technical discussion. The diagram shows an FXO interface to the
Public Switched Telephone Network (PSTN). Voice quality issues generally come
up in gateways with analog FXO interfaces. The issues are often the result of
the variations of the cable plant in combination with the hybrid. The hybrid
performs two-wire to four-wire translation. The voice port can also be a DID
interface to the PSTN because the port is also a long-haul trunk interface.
However, FXO interfaces have a more dominant presence in field installations of
long-haul analog voice. FXS interfaces, on the other hand, typically exhibit
acceptable quality of service. FXS interfaces usually connect to short-distance
premises wiring instead of miles of telco cable, as is typical of FXO
interfaces.

After the installation and configuration of a voice router, users
sometimes notice audio quality behavior which differs from their experience
with a traditional time-division multiplexing (TDM) voice network.� Audio
problem reports can include click noises, hiss, audio volume level issues,
chop, one-way or no-way audio, or echo.� You can find these problems on voice
routers that employ either digital voice port connectivity to a voice switch or
analog voice port connectivity. But, in practice, the analog voice port
connection more often causes complaints from users.� In most situations, you
can eliminate audible voice quality issues if you properly understand the
sources of these problems and the subsequent tuning of the packet voice
network.� You can prioritize voice packets over data traffic. You can eliminate
or mitigate clocking mismatches. You can adjust signal levels. And, in the case
of analog voice ports, you can considerably reduce echo and mitigate other
problems if you properly match impedance to the telco line conditions.

The next figure highlights some aspects of Cisco FXO voice port
operation which influence the overall voice quality that a user experiences.
The call in this scenario is a VoIP call between a Cisco voice router and a
PSTN party.� These factors affect voice quality:

The input gain, output
attenuation, and
impedance
settings of the port

The echo canceller, which includes cancellation performance,
double-talk detection performance, and the nonlinear processor (NLP)
algorithm

The transmit level that the CO
provides

A detailed discussion of each area of concern is beyond the scope of
this document. However, note that at the interface between the Cisco FXO voice
port and the PSTN cable plant is an impedance that attempts to match the
channel as the PSTN presents it.

The cable plant that is attached to the Cisco FXO interface presents
impedance that is primarily a function of cable length and cable gauge. There
are secondary aspects of the cable plant which affect impedance, but these
aspects are beyond the scope of this document. These aspects include the
dielectric material of the cabling, temperature, twist pitch, mixed gauge
lines, bridged taps, CO terminating impedance, voice frequency repeaters, and
loading coils.

An RJ-11 Tip and Ring conductor pair is a very simple transmission line
between your CO and the voice port on the Cisco voice router. Over the length
of the transmission line, you have a model of distributed resistance,
distributed capacitance, and distributed inductance. In the end, from the
perspective of the voice port on the Cisco voice router, you are mating with an
interface that you can model as an impedance Z composed of a
real resistance R summed with a frequency-dependent
complex-valued reactance X:

Z(f) =
R+jX(f)
= √(
R2+X2(f)
)
ejarctan(X(f)/R)

Note: f is the frequency in hertz.

X(f) is dependent on the
capacitance and inductance on the line and is a function of frequency
f.�Other frequencies differently affect each spectral
component of a voice band call. The varying nature of
Z(f) causes this difference, with both a
change in the magnitude of the signal as well as the phase.

You want to match the voice port impedance setting Z'
with this aggregate transmission line impedance Z.You
calculate the reflection parameter Rf,
which indicates how good the match is, with this equation:

Rf = ( Z –
Z' ) / (Z + Z' )

The better the match, the smaller the magnitude
|Rf| tends toward zero. Also with a
better match, less signal reflects back in either signal direction. If you have
a perfect match, you have no reflected signals whatsoever.�This is almost
impossible to achieve over all frequencies f, so there is
always some mismatch. Therefore, there is always some reflection of speech
energy, which can cause some echo.�Cisco analog FXO implementations have a
finite selection of impedance settings. You cannot expect any setting to match
the telco line impedance exactly. There can be a setting, however, which offers
the best impedance match. This setting offers the best hybrid performance.�The
best match is a setting that provides both of these
parameters:

The highest THL, which is the least amount of hybrid
echo

The minimum receive loss, which is the highest receive
level

Also, you can identify nobest
match when hybrid performance results are mixed or about the same.
Under these conditions, you can use listening tests and comparisons of voice
quality to choose the Cisco FXO interface impedance setting.

The available
impedance
values under Cisco analog FXO, FXS, and DID voice ports are
600r, 600c, 900c,
complex1, complex2,
complex3, complex4,
complex5, and complex6. When you set one of
these values, you try to match the telco line as closely as you can. Choose
either:

Settings which are fully resistive

An impedance which is mostly resistive

An impedance which is mostly
reactive

Choose whatever seems to work best to reduce reflections on the
line.

The
impedance
options complex4 and complex6 are compromise
networks that the EIA RS-464 standard proposed.� These networks have fairly
consistent performance characteristics over a large range of telco loop lengths
with an output impedance of 600 ohms.� The
impedance
option complex5 is an optimized configuration for 12,000 feet
of 26 American Wire Gauge (AWG) cabling. The complex5 option
changes the output impedance to more closely resemble the line.

Use these recommendations as general guidelines:

0 to 5,000 feet—Use 600r, or match the voice port
impedance setting to the impedance specification of the peer equipment.

In North America, for example, the typical impedance rating of a CO
or PBX analog trunk port is 600r. But in other parts of the world, the
impedance rating can be 900c.

5,000 to 10,000 feet—Use complex4.

10,000 to 15,000 feet—Use either complex5 or
complex6.

The complex4 and complex6 settings
have slightly less power transfer loss than complex5.�If there
are signal-level issues to consider, choose the complex6
setting over complex5.

Cisco IOS Software Release 12.3(11)T introduced tools which you can
apply methodically to help ascertain the best match
impedance setting for an analog voice port.� In releases earlier than Cisco IOS
Software Release 12.3(11)T, empirical tests generally determined the choice of
an impedance setting. These empirical tests involve the trial-and-error method,
which can be frustrating and inconsistent.� The end user and an engineer from
Cisco Technical
Support usually performed the test on a conference bridge. They worked
during a maintenance window for up to several hours.� With the new test tools
in Cisco IOS Software Release 12.3(11)T and later, the end user can
independently complete this voice port impedance tuning in a short amount of
time. The end user only needs to engage
Cisco Technical
Support when problems persist.� The two test tools that this document
discusses are:

(*) See the Additional Notes section of
this document for important notes regarding support for the THL Tone Sweep
feature on the Cisco 1751 and 1760 voice platforms.

Both test methods involve the placement of test calls through the
analog FXO, FXS, or DID voice port, between a party on the IP network and
another party. The test injects test tones of known signal strength and
frequency out the analog port.� Then, the test inspects the return signal and
tabulates the Echo Return Loss (ERL) in order to provide a channel profile of
ERL versus frequency.� A higher ERL at any given frequency point is better.�
Expect the channel profile to show good ERL levels at low frequencies and
across the voice band. The ERL levels then start to taper off at higher
frequencies.� You perform this test for each available impedance setting. The
test selects the setting that provides the best channel profile as the
best match impedance for that voice port and that telco
line. �For both test features, the value that indicates the suitability of the
channel profile is the arithmetic mean of the ERLs over all tested frequencies
for a single impedance setting.� This formula illustrates:

ERLavg = (ERL1 +
ERL2 + … + ERLN ) / N

Note: ERLi = ERL measured at the
ith frequency. N is the total number of tested
frequencies.

The best match impedance for the voice port is the
impedance setting that yields the highest value of
ERLavg.

Cisco IOS Software Release 12.3(11)T introduced the Original Tone Sweep
method of determination of the best match impedance. The
method is also available in Cisco IOS Software Releases 12.3(14)T, 12.4(1), and
later.� The method requires some manual work by the tester to complete the
suite of tone tests.� Specifically, you must manually change the impedance
setting under the voice port for each new battery of tone tests. You
administratively issue the shutdown command and the
no shutdown command on the voice port to have the
change take effect. Then, you place a new test call from the FXO/FXS/DID voice
port and execute the battery of tone tests again.� You repeat the process for
each different impedance setting that the voice port allows.

These are the steps to complete:

Important: Disable ECAN under
the voice port of interest.

Issue the no echo-cancel enable
command.

Note: Be sure to administratively issue the
shutdown command and the no
shutdown command on the voice port so that the change takes
effect.

Place a call over the FXS/FXO voice port of
interest.

Issue the show voice call summary
command to verify the connection of the call.

Note: The party out in the PSTN or on the PBX side of the voice port
must be a “quiet termination”. If necessary, mute this phone so that it is not
a source of audio.

Execute the tone sweep test for this voice
port.

Calculate the value of ERLavg for this
impedance setting.

Change the impedance setting under the voice port of
interest.

Note: Be sure to administratively issue the
shutdown command and the no
shutdown command on the voice port so that the change takes
effect.

Repeat steps 2 through 5 until you have exhausted all possible
impedance settings under the voice port of interest.

Look over your collection of ERLavg to find
the highest value.

The impedance setting to which this value corresponds is the
best match impedance under the voice port of
interest.

Here is an example of the sweep in action for two impedance settings,
complex1 and complex2:

Choose complex1 as the best match impedance
because complex1 has the higher average ERL of 21.16.

This Original Tone Sweep method to determine the best
match impedance setting can be cumbersome. The method is especially
cumbersome in a live production environment where other parties compete for use
of the same voice port that you wish to use as your reference port for the
tests. With this method, you must place multiple calls over the same voice port
to a “quiet termination” point out in the PSTN. You must change impedance
settings manually between each set of tests. If a production call happens to
seize the target voice port before you can initiate the next test sweep, the
user likely hears echo. The echo occurs because you have disabled ECAN on that
voice port.� Despite these drawbacks, this test method is superior to the
trial-and-error method that preceded this feature.

In order to ease the administrative burden of the Original Tone Sweep
test method, Cisco IOS Software Releases 12.3(11)T6, 12.3(14)T3, and 12.4(1)
introduced the THL Tone Sweep test method for the Cisco 2600XM, 2691, 2800,
3640, 3660, 3700, and 3800 Voice Router platforms. The feature was later
extended to the Cisco 1751 and 1760 platforms in Cisco IOS Software Releases
12.3(14)T6, 12.4(3b), 12.4(5a), 12.4(7), 12.4(2)T3, 12.4(4)T1, and 12.4(6)T, as
well as the Cisco IAD2430 and VG224 platforms in Cisco IOS Software Releases
12.4(7) and 12.4(6)T. This test feature allows the evaluation of all available
impedances for a single test call to a quiet termination point out in the PSTN.
You do not need to manually disable ECAN on the voice port under test. The test
feature switches impedances automatically for the tester. The test feature
calculates the arithmetic mean ERL and reports the mean for each channel
profile at each impedance setting. Then, at the end of the test, the feature
specifies the best match impedance setting. This test
feature is simple to use and requires minimal supervision.

These are the steps to complete:

Place a call over the FXS/FXO/DID voice port of
interest.

Issue the show voice call summary to
verify the connection of the call.

Note: The party out in the PSTN or on the PBX side of the voice port
must be a “quiet termination”. If necessary, mute this phone so that it is not
a source of audio.

Execute the tone sweep test for this voice
port.

The THL Sweep test feature automatically calculates the value of
ERLavg for each impedance setting. The feature reports
the setting that yields the highest value of ERLavg at
the conclusion of the test.� This setting is the best
match impedance setting to use under the voice port of
interest.

As opposed to a trial-and-error method, the Original Tone Sweep and THL
Tone Sweep test methods provide a consistent means to evaluate the worthiness
of a particular impedance setting when used with the telco channel.� While you
perform the tests, be aware of these points:

Keep the test methodology as consistent as possible.

If you use the Original Tone Sweep method, use the same party as the
“quiet termination” in the PSTN for each set of tone sweeps at each impedance
setting.� This choice keeps the path between the voice port and the termination
point the same.

On voice routers with many analog FXO/FXS voice ports, you do not
necessarily need to apply the tone sweep tests to every voice port.

If time is in short supply, you can test a single voice port and use
the result as representative of the behavior of all the voice ports from that
same telco provider.� In the majority of cases, this assumption is correct
because the wiring path is most likely the same for all ports. For best results
however, each voice port should be tested and tuned individually.

After selection of the best match impedance
setting, perform further tuning of the voice ports as necessary in order to
eliminate any residual audio problems.

Most likely, you need to tune the input
gain and output attenuation settings
in this case.

The best match voice port impedance setting
applies to the direction from the Cisco voice router toward the PSTN.�

After you set this best match voice port
impedance, there is no guarantee that the ERL performance of the channel from
the perspective of the PSTN toward the Cisco voice router will be symmetric and
provide the highest possible ERL profile in this direction.� Gauge the overall
voice quality in both directions and decide whether to tune voice port
parameters further. Engage Cisco Technical Support,
if necessary. In the majority of cases, the qualitative perception of voice
quality is a noticeable improvement after you set the voice port impedance to
the best match value. Users in the field have reported
this improvement.

The Cisco 1751 and 1760 Voice Router platforms use the PVDM-256K-4,
PVDM-256K-8, PVDM-256K-12, PVDM-256K-16, and PVDM-256K-20 DSP card products for
voice signaling and media. These PVDM-256K-* cards use the
Texas
Instruments C549 DSP. Due to DSP firmware and processing power
limitations when operating in Medium-Complexity (MC) Codec mode, the THL Sweep
feature on the 1751/1760 Voice Router platforms only functions reliably when
the DSPs are set for High-Complexity (HC) mode. By default, 2-port Voice
Interface Cards (VICs) such as the VIC-2FXS, VIC2-2FXS, VIC-2FXO, VIC2-2FXO,
VIC-2E/M, VIC2-2E/M, and VIC-2DID are assigned to a single C549 DSP operating
in HC mode for its signaling and media resources. On the other hand, 4-port
VICs such as the VIC2-4FXO and VIC-4FXS/DID are assigned to a single C549 DSP
operating in MC mode to make the most optimum use of available DSP resources.
As a result the THL Sweep feature on the 1751/1760 often fails when applied to
the 4-port VICs, and you can potentially see this error: